1. Field of the Invention
This invention relates to material handling structures having a vertically pivotal boom of extended horizontal length and more particularly to apparatus for counterbalancing and stiffening such structures.
2. Prior Art
Material handling structures having vertically pivotal booms of extended horizontal length are conventionally counterbalanced by weights disposed diametrically opposite the boom. In some of these structures, the counterweight is mounted on a pivoted strut. A cable or other tension member is connected between the counterweight and a point well out on the boom. The intermediate portion of the tension member is held aloft by a king post truss or other structure such that a vertical component of the tension member force counterbalances the boom. In such structures, a horizontal component of the tension member force applies a longitudinal force to the extended length boom. As the boom is raised and lowered, the counterweight moves in the opposite direction to maintain the counterbalancing force. One advantage of such an arrangement is that the structure remains properly counterbalanced even if the boom should come to rest on an object or the luffing cables or mechanism should fail.
Structures of the above type are often used in stacking and reclaiming bulk materials, such as coal and ores, and may have booms 100 to 200 feet in length. A digging device, such as a bucket wheel mounted on the end of the boom, deposits bulk material on a conveyor which extends the length of the boom. For stacking bulk material, the direction of the conveyor is reversed. The forces developed by the bucket wheel at the end of the boom and the weight of the bulk material along the length of the conveyor impose a sizable live load on the 100 to 200 foot boom which causes it to bend. The bending of the boom produces an uneven bench in the stack being reclaimed as the boom becomes fully loaded. This is considered very undesirable by users of the machine. Under some circumstances, the bending can cause the bucket wheel to dig in and stall.
The counterweight acting through the cable or tension bar connected well out on the boom tends to counteract to some extent the droop produced by the live load. Although it may seem suitable at first to increase the size of the counterweight to reduce the droop further, this is not a satisfactory solution since the larger counterweight causes the boom to bend upward upon removal of the live load and, therefore, the total deflection remains the same. In addition, if the size of the counterweight is increased to counteract the live load, the buckle strength of the boom can well be exceeded when the liveload is removed.